The invention relates to a superconducting wire, containing NbTi superconducting material and Cu, comprising a multiplicity of hexagonal elements, which have an at least approximately hexagonal outside contour, as seen in cross-section perpendicular to the longitudinal direction of the superconducting wire.
Such a superconducting wire is disclosed in U.S. Pat. No. 5,088,183 A.
Niobium-titanium (NbTi) is an important superconducting material, which, due to its good plastic deformability (both as a final material and with respect to its precursors), can be deployed in a variety of ways, for example, in superconducting magnet coils or in superconducting cables. The NbTi superconducting material is typically integrated in a superconducting wire, wherein a multiplicity of NbTi filaments usually extend in the longitudinal direction of the superconducting wire.
The wire manufacturing process usually includes, as part of a drawing process, plastic deformation into a hexagonal outside cross-section in order to simplify the bundling process of individual NbTi filaments (or also other structures and intermediate structures). The resulting hexagonal elements can then be very compactly disposed, for example, in a copper enclosing tube, cf. U.S. Pat. No. 5,088,183 A referred to above.
The superconducting properties of NbTi are only achieved at especially low temperatures, below approx. 9 K, so that the superconductor must be cooled, for example, with liquid helium. Moreover, the superconducting condition cannot be maintained if the magnetic field strength or the electric current density is too high.
NbTi superconducting wires are typically stabilized with copper (Cu), usually with a high degree of purity. The copper runs parallel with the superconducting filaments in the superconducting wire; for example, the copper can constitute a matrix for superconducting NbTi filaments. Copper is a good heat conductor, through which effective cooling of the superconducting NbTi filaments can be achieved. Furthermore, copper exhibits high electrical conductivity. If superconductivity in the superconducting wire is lost locally, the copper provides a parallel electrical current path, limiting a rise in electrical resistance and the associated heat generation. With effective cooling, superconductivity can be restored to the local, normally conducting region in the superconductor (stabilizing function). Even if superconductivity cannot be recovered, the copper protects the superconductor from damage (“melting”) by limiting the increase in resistance and thus limiting heat generation, while also efficiently removing the resulting heat (quench protection). Moreover, the copper improves the mechanical strength of the superconducting wire.
Copper is not only an expensive material, it is also relatively heavy with a density of approx. 8.9 g/cm3, which is undesirable for many applications. Alternative materials for stabilizing NbTi superconductors are therefore sought that can at least partially replace copper. Pure aluminum (Al) would be a suitable material in terms of heat conductivity and electrical resistance, and is less expensive than copper and, with a density of 2.7 g/cm3, considerably lighter.
WO 2008/121764 A1 proposes a NbTi superconductor structure with a Cu sleeve, an Al core and multifilament NbTi rods disposed in a ring between them.
While Cu and NbTi are both relatively hard and behave in a similar way when plastically deformed, Al is much softer than NbTi and Cu. When wire containing Al structures is drawn, in particular, an Al core, cracks that run transverse to the longitudinal direction of the wire have been observed in the NbTi superconducting wire. A wire containing cracks is of no use for superconductor applications.
U.S. Pat. No. 5,189,386 discloses a superconductor structure in which an Al core is divided into six sectors by Cu—Ni alloy layers. This structure is relatively complex.
U.S. Pat. No. 4,652,697 describes a superconductor structure in which three Al wires and four multifilament wires are bundled, for example, with NbTi filaments and a copper matrix. One of the multifilament wires is disposed in the center of the superconductor structure. The structure is held together with PbSn solder. It is difficult to process solder in wire production and, as an additional material, it can additionally aggravate the problem of crack formation.
JP 09 282953 A describes a Nb3Al superconductor wire in which Nb3Al filaments are embedded in a copper matrix. Embedded additional structures made of Nb or Ta can be used for strengthening. In one embodiment, additional structures with an Al rod in a Cu tube are proposed.
The object of the invention is to propose a stabilized NbTi superconducting wire, which has low weight and which, in the manufacturing process, especially during wire drawing, reduces the risk of crack formation.